Countless words have been spent to make a point about USP and explaining it to noob. Here is one more effort.
What is USP?
USP or Unique Selling Proposition is the thing that makes you special. Out of all the people who are blogging about blogging, what makes you special?
Sometimes Often, you just can't put it in words - it's just an experience. Apple's eye for beautiful design is more of feeling work than theory.
How to Find One?
It's a voyage, really, many people lose themselves in quest to find one because often they don't realize that they already have a USP.
Often your USP is you (you are unique after all), the other times it's the way you talk, they way you walk (models) or cook, it can be anything. If unsure, ask your customers what they like about you. And ask your non-customers what they hate about you.
The best way is to know all you can about USP so you'd know when you notice yours. Here are some resources that could help you discover it: